An optical fiber is produced by heating and melting an optical fiber preform (hereafter referred to as a preform) using a dedicated drawing furnace, by drawing a glass fiber and by applying protective coating to the external surface thereof. The drawing furnace has a configuration in which a susceptor into which the preform is inserted is disposed inside the furnace body thereof, a heating apparatus disposed outside the susceptor heats the susceptor, a glass fiber is allowed to droop from the lower end of the heated and melted preform, and the glass fiber is pulled out through the lower outlet of the susceptor. The susceptor is usually made of heat-resisting carbon; however, a gas (hereafter referred to as an inert gas or the like), such as a rare gas, for example, argon (Ar) or helium (He), or nitrogen (N2) is supplied into the susceptor to prevent the oxidation of the susceptor.
The inert gas or the like having been supplied into the susceptor flows from the upper side to the lower side of the susceptor in many cases; in such a case, the gas is discharged from the lower side of the susceptor to the outside together with the glass fiber having been drawn from the preform. In the case that a thick preform is drawn, the space around the neck-down region of the preform becomes large, and the temperature distribution of the gas flowing in the space becomes non-uniform; as a result, the variation in the diameter of the glass fiber to be drawn tends to become large. If the variation in the diameter of the glass fiber becomes larger, a problem, such as a greater connection loss at the time of connector connection, may occur. To solve this problem, a method for suppressing the variation in the diameter of the glass fiber by using He gas having high thermal conductivity as an inert gas or the like and by making the temperature distribution uniform is used in some cases. Furthermore, a protective pipe (also referred to as a lower chimney or a lower extension pipe) is provided at the lower section of the susceptor to keep the glass fiber away from the outside air immediately after the drawing, thereby suppressing the variation in the diameter of the glass fiber.
However, since He gas has high thermal conductivity, in the case that He gas is used inside the protective pipe, the glass fiber is cooled quickly. In the glass fiber having been heated and melted, the atoms and molecules inside the glass vibrate due to the thermal energy thereof, and the arrangements of the atoms and molecules are disordered, whereby the atoms and molecules being in the disordered states are rearranged and the structural relaxation of the glass is promoted while the glass is cooled, and equilibrium is reached at a predetermined temperature and the glass is frozen and solidified. The equilibrium temperature serving as an indicator of the disorder in the glass structure is also referred to as fictive temperature; in the case that the glass is cooled slowly, the disordered states of the atoms and molecules inside the glass are relieved gradually, and the fictive temperature is shifted to the lower temperature side. On the other hand, if the glass is cooled quickly, the glass is frozen and solidified while the rearrangements of the atoms and molecules inside the glass are disordered, and the fictive temperature is shifted to the higher temperature side.
As described above, in the case that He gas is used, the glass fiber being in the heated and melted state immediately after the drawing is cooled quickly inside the protective pipe by He gas having high thermal conductivity, and the glass is frozen while the atoms and molecules inside the glass are disordered; as a result, the fictive temperature is high and the Rayleigh scattering intensity of the optical fiber cannot be reduced; hence, it is said that transmission loss increases.
To cope with the above-mentioned problem, for example, Patent Document 1 discloses a method for suppressing the glass fiber from being cooled quickly by providing a gas mixture layer in which He gas is mixed with a gas having low thermal conductivity between the neighborhood of the outlet of the susceptor and the protective pipe.
Patent Document 2 discloses a method for slowly cooling the glass fiber by introducing the drawn glass fiber into a slow cooling section and by keeping the glass fiber at room temperature or heating the glass fiber using Ar gas serving as a temperature adjustment gas.
Patent Document 3 discloses a method for decreasing transmission loss by performing heat treatment (slow cooling) when the glass fiber is drawn from the preform to set the fictive temperature in a predetermined range and by decreasing the residual stress (tensile stress) of the glass fiber in the direction from the inside to the outside of the clad layer to decrease the difference in the distribution of the residual stress in the radial direction.